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Name: Ida
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Question:
What is the relation between the angle of the noon sun and the quality of solar radiation received per square centimeter at the outer edge of the atmosphere? What causes the intensity and duration of solar radiation received at any place to vary throughout the year?



Replies:
Ida,

The angle of the noon sun will not affect the average intensity so much as where the intensity is greatest. When the angle of the noon sun is more northward, the northern hemisphere receives most of the radiation. This is why we experience summer at this time. When the sun is very much toward the south, the radiation intensity is greater in the southern hemisphere. Although tilt is not a direct affect to intensity, distance from the sun and sun activity are very important. The Earth's orbit is not a circle. The Earth is closest to the Sun in January. It is furthest six months later. The entire Earth receives a greater radiation during January. It receives less radiation during July. In addition, the Sun itself is not constant: sometimes the sun emits more radiation, sometimes less. This can also affects the amount of radiation that reaches our planet. I suspect the only measurable link between tilt and radiation intensity is the fact that Sun appears furthest South when the Earth closest to the Sun. Thus, more radiation strikes the Earth when the Sun is furthest South. A lower radiation intensity strikes the atmosphere above Illinois, but more strikes the atmosphere of entire planet.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


Ida,

these are actually quite distinct questions, and there are many factors involved.

"What is the relation between the angle of the noon sun and the quality of solar radiation received per square centimeter at the outer edge of the atmosphere?"

I am not sure what you mean by 'quality'. If you mean _quantity_, the simplest relationship is that the radiation is lowered, approximately proportionally to the sine of the angle of the light. The more extreme the angle, the less the apparent intensity.

If terms of _quality_ of radiation (e.g. relative intensities of different wavelengths), I am not sure angle would play a role (other than in some negligible quantum effects). The earth's magnetic field also plays a role in deflecting some radiation (which would affect radiation quality) and the magnetic field does have an orientation, but that orientation is not independent of the earth's orientation (and therefore not strictly consistent with angle of the surface)

You specifically say the outer edge of the atmosphere, but I also want to discuss the role the atmosphere plays, since it affects _quality_ as well. In terms of _quality_, the atmosphere filters out / reduces some wavelengths of light (UV, for example), and it also redirects different wavelengths of visible light (read about Rayleigh scattering). Especially for Rayleigh scattering, the scattering is very much dependent on the angle of the light.

"What causes the intensity and duration of solar radiation received at any place to vary throughout the year?"

The biggest impact on intensity and duration of solar radiation is cloud cover -- cloudy areas get less sun. The intensity of the sun changes as well, but in general those changes are not as rapid as cloud cover changes.

There are also differences as the times of sunrise and sunset change. (nau)

Hope this helps,

Burr Zimmerman



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